Pluralism and the Strains of Commitment

[Warning: This post assumes a lot of background, and may not be generally accessible.]
I've been re-reading bits of Justice as Fairness to try to nail down Rawls' take on the relationship between the difference principle and the value of the basic liberties. But I got sidetracked.
The story on the development of the doctrine of “justice as fairness” is that Rawls saw that he initially grounded the argument for the stability of his ideal system on a particular comprehensive conception of the human good — one that is neither all that broadly shared, nor rationally mandatory. The argument in A Theory of Justice failed to take into account the inevitable pluralism in such conceptions in a free society. So Rawls famously changed the structure of his argument in Political Liberalism to better accomodate the unavoidable fact of pluralism in free societies.
It has been frequently pointed out that it's pretty remarkable, and suspect, that this rather fundamental change in the argument entailed no really substantive change in Rawls's conclusions. And that is remarkable, and suspect. Rawls just never does get his head around real pluralism, and his account of stability always seems to revert to the assumption that people brought up within a just social order will end up believing and valuing the same thing when it matters to the argument.
What I have in mind is his brief discussion of the strains of commitment in Justice as Fairness. The question is why won't the rich always try to renegotiate the principles that govern our basic social institutions to their benefit? If they do clamor for more, the principles won't last, and so won't be stable, which they must be if they are to be adequate principles of justice. So why won't the relatively rich try to get a better deal, if raised under the right institutions?
First, EVERYBODY shares an idea about the point of society and political institutions. Impossible. That idea — free and equal people engaged in mutually advantageous cooperation — entails a certain idea of reciprocity. Sure. And that principle of reciprocity is the difference principle, Rawls says. Remember, the idea is that all rich people raised under just institutions will think this. Unlikely. But it gets more implausible the more he drills down:

We also suppose that in addition to the reason which all have [that the difference principle is the principles of reciprocity implied by the abstract political conception everyone will allegedely share], the more advantaged have a second reason … The point here is that the more advantaged see themselves as already benefited by their fortunate place in the distribution of native endowments, say, and benefited further by the basic structure (affirmed by the least advantaged) that offers them the opportunity to better their situation, provided they do so in ways that improve the situation of others.

You know you're in trouble if your argument (intended to be adjusted to the inevitably roiling pluralism of a free society) depends on all-but-universally-shared ideas about “the distribution of native endowments.” A bit earlier, Rawls notes that “this idea of reciprocity is implicit in the idea of regarding the distribution of native endowments as a common asset.” That's also part of what, come the reign of justice, we'll all understand.
I'm sorry. Even granting Rawls' badly undermotivated framework stipulation that the strains of commitment cannot be expressed through emigration or capital flight, this is a total failure if the aim is to take reasonable diversity of thought seriously. These passages read like a reductio of the attempt to reconcile justice as fairness with the fact of reasonable pluralism. By Rawls' own account, JAF seems to have no hope of passing the compliance/stability test without simply positing a level of agreement that makes a mockery of the whole idea of reasonable pluralism.
Remember, Rawls' project is to outline a realistic utopia — a society that could really exist given actual human nature. So his stability arguments amount to predictions about, among other things, the beliefs and desires that would prevail among people brought up under institutions that satisfy his two principles of justice. The principle with the very highest priority here is an inviolable right to free thought and expression. And Rawls' prediction is, what? That in that kind of society — in which freedom of thought, speech, conscience, etc. are paramount — rich people won't try to get a bigger piece because they will all agree, more or less, that the “distibution of native endowments” is a common asset, that they've got it plenty good, and there could be no justification for wanting more.
We'll never be in a position to see this prediction play out, but knowing what we know about the way actual human minds work, I would bet the farm against it. Even when he weakens his Kantianism, Rawls leans hard on it, and is undone by it. For all the talk about pluralism, he's really depending on some assumptions about the universal structure of the “two moral powers” — rationality and the moral sense — that make his yearning for a kind of stability that is more than a fragile, contingent modus vivendi seem plausible. But it just isn't plausible.